This invention relates to devices for attaching downhole tools to pipe, tubing, casing, or the like. In particular, the invention relates to a stop collar or limit clamp.
Downhole tools such as centralizers are typically attached to casing by a stop collar or, limit clamp. Stop collars or limit clamps have taken on many styles including: hinged friction collar, hinged collar with set screw, hinged collar with dogs, and slip collar with set screws.
The hinged collar has two semicircular bands which are joined at one end by a hinge. At the opposite ends from the hinge, the semicircular bands have a flange through which a bolt extends between the two flanges. Thus, the hinged style stop collar is attached to a pipe by spreading the semicircular bands wide enough to receive the pipe. Rotating about the hinge, the semicircular bands are close together until the flanges are proximate one another. A bolt is then inserted through the flanges and tightened. As the bolt tightens, the flanges are drawn closer together so as to squeeze the collar about the pipe.
The hinged collar with set screws also comprises two semicircular bands which together surround a pipe. In this case, however, both ends of both semicircular bands have a hinge. The hinge is made up of corresponding eyelet pieces which are joined by a pin. Thus, the collar is attached to a pipe by placing the semicircular bands on opposite sides of the pipe and mating the hinge eyelets at the ends of the bands. With the hinge eyelets properly mated, pins are inserted into the eyelets. The semicircular bands also comprise set screws which are used to tighten the collar on the pipe. The set screws extend in a radial direction through the bands toward the pipe. Any number of set screws may be used to secure the collar to the pipe, but six set screws equidistant from each other is typical.
Hinged collars with dogs are again made of two semicircular bands which mate with each other to extend about the circumference of a pipe. Rather than eyelets, two ends of the semicircular bands are joined by interlocking fingers. The opposite ends of the bands have flanges through which a bolt extends. As with the hinged style collar, as the bolt is tightened, the flanges are drawn closer together so as to squeeze the bands around the circumference of the pipe. This collar also has several dogs which extend radially through the bands to provide protrusions or bulges on the interior of the bands for engagement with the casing. As the bolt is tightened and the bands are squeezed about the circumference of the pipe, the dogs firmly engage the outer surface of the pipe.
Slip on collars with set screws are made of a single circular band, rather than two semicircular bands. This circular band is slipped over the end of a pipe and moved longitudinally along the pipe to the position at which it is to be secured to the pipe. Once the correct position is achieved, set screws which extend radially through the band are screwed inwardly toward the pipe until they firmly secure the collar to the pipe. Any number of set screws may be used with a slip on collar but 5–6 set screws equidistant from each other are typical.
A primary application for stop collars or limit clamps is to secure dual-ring centralizers, single-ring centralizers (bow spring and rigid) or other casing attachments to casing outside surfaces. In particular, flushline casing centralizers are connected to the casing by these type stop collars. Centralizers typically comprise two rings connected to each other by bow springs. With each of the stop collar or limit clamp designs noted above, there is a minimum required gap between the casing outside diameter and the hole inside diameter. Typical stop collar configurations have the following holding capacities and outside diameters (OD) on the noted pipe sizes:                4½–17,778 pounds force with a positive OD of 6.00 inches.        5½–24,941 pounds force with a positive OD of 7.00 inches.        7–35,000 pounds force with a positive OD of 8.50 inches.        9⅝–39,000 pounds force with a positive OD of 11.125 inches.        
In slim hole casing programs, the gap between the OD of the centralizer and the inside diameter of the wellbore is generally near 0.375 inches–0.625 inches per side. Where the stop collar is in the middle of a dual-ring, bow spring centralizer, the thickness of the bow spring must also be added to each side to determine the actual minimum restrictions through which a dual-ring centralizer must pass. Smaller gap restrictions have led to the increased use of integral rigid centralizer subs which do not require limit clamps at all. Flushline casing connections, in particular, present annulus gap sizes too small for conventional stop collars or limit clamps.